The recovery of oil from oil shale deposits is most commonly effected in above ground retorting operations, that is from oil shale brought to the surface and retorted in a nearby or on-site operation. Two procedures are now generally used, one the "direct method" and the other the "indirect method". These processes may be continuous or batch operations. The present invention is for the indirect method preferably using a continuous traveling grate retorting procedure but applicable also to other indirect processes.
In the direct process gas produced from the shale in the retort is mixed with air and recycled into the retort at a location along the traveling grate where volatiles have been largely removed in preceding stages of the distillation but where product gas derived from the distillation process is mixed with excess air and burned in the retort oxidizing the residual carbon in the shale. Heat so generated by combustion on the grate is used in the earlier stages of retorting and thus is recuperated resulting in the production of more product gas.
In the direct process, product gas in excess of what it burned is produced but it is combined with the products of combustion. It is what I term a "low value gas", too lean for pipeline gas or hydrogenation of the shale oil for use as crude refinery stock, and must be burned locally in plant operations, as for example, the generation of steam to operate a turbine driven electric generator.
In the indirect process, the major portion of the product gas is burned in a heat exchanger to heat recycle product gas. The hot non-oxidizing product gas, in turn, heats the shale on the grate to release the oil and generate the fresh product gas. In this closed cycle shale oil is recovered but the major portion of the product gas is consumed as fuel for the heat exchanger and exhausted from the heat exchanger as flue gas. The heat exchanger used for this purpose may be incorporated in the traveling grate itself or be separate from but close to the traveling grate. For simplicity of disclosure, the heat exchanger is shown in the accompanying drawings as an external unit adjacent the retort, but without exclusion of a heat exchanger comprising an integral section of the traveling grate retort.